Structural complexity shapes the behavior and abundance of a common herbivorous fish, increasing herbivory on a turf-dominated, fringing reef

Abstract

Coral-dominated reefs typically have abundant refuges and limited resources for herbivores; conversely, algal-dominated reefs have abundant resources but often have limited refuges. This difference results in a trade-off between risk and reward for herbivorous fishes. However, disturbances that kill coral but retain topographic complexity can decouple this tradeoff, challenging this paradigm. Many fringing reefs in Moorea, French Polynesia are algal-dominated with heterogeneous structural complexity likely driven by disturbances associated with crown-of-thorns predation and typhoons, which occurred between 2006 and 2010. Short turfs are a good recruitment surface for coral larvae, and thus has a greater potential to recover to coral dominance than long turfs. To explore how variation in complexity impacts the behavior of herbivorous fishes, we compared abundance, residence time, and time budgets of Acanthurus nigrofuscus, a common herbivore species, in paired high versus low complexity areas. We chose areas in close proximity to maximize the likelihood fish could choose between complexity types. We also measured turf height in high versus low complexity plots. Fish were ~3× more abundant, spent roughly double the time in, and budgeted more than 4× as much time to hiding in high complexity reef areas. Although, on average, individual herbivores spent the same proportion of time foraging in both areas, their greater abundance and longer residence time suggests their local herbivory rates could be 7.5 times higher in high complexity areas. We also found turfs in high complexity plots were roughly 75% the height of turfs in low complexity plots. This provides some evidence for stronger overall herbivory pressure in high complexity areas, albeit differences in absolute turf height were modest (~0.5 mm). Combined, these results suggest reefs that retain structural complexity following disturbance may be more likely to recover due to the higher localized herbivory rates resulting from uncoupling of the tradeoff between structural complexity and resource limitation.